I have a co-sleeper with my son and tried sitting next to him and telling him it was night night, etc., patting the bed for him to lay down to go sleep.
The first night it was 15 minutes; he fussed and eventually I laid him down and he went off to sleep. But the second night well he cried for almost 20 minutes.
I was trying to be as gentle as I could and loving. My husband couldn't take it anymore and I nursed him to sleep and he went off in about 5 seconds as he was very tired.
My first question would be, did you do any sort of nighttime routine? Did you nurse him before laying him in bed? How old is he?
If he's a newborn, it just may not be realistic to get him to sleep without nursing at this point. Tiny babies like to eat and then sleep. If they're hungry, they're awake and want to eat. Then they tend to fall back asleep right away.
If he's older, look at his routine. I have found that a strong routine that stays the same each night is important.
I also think it's important to nurse your baby until he's nice and full. If he starts to drift off, you can break the latch – he's probably done nursing.
What has worked for me with Galen is to break the latch as he's starting to drift off to sleep, then go through the rest of his routine – a warm bath, getting dressed in jammies, then rocking with me.
If I just go from breaking the latch and putting him to bed he's cranky.
I really think a good routine sets up expectations of bedtime and babies quickly adjust to this.
If your baby is older (not a newborn), then I recommend you read through this free Baby Sleep Report for a lot of practical ideas on helping establish a sleep routine for your baby.
You can also look into the Baby Sleep Solution program by the same author as the report. He goes into a lot of detail on specific techniques to help your baby get to sleep gently and without crying.(Hint: Would you like practical, no-nonsense, really-help-you-get-more sleep tips on handling nap time and bedtime with your little one? Check out the baby sleep crib sheet - no crib required - and get better sleep tonight. Get it here.)